1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine provided with a catalyst capable of reducing nitrogen oxides (hereinafter, NOx) under oxidizing conditions and in the presence of hydrocarbons.
2. Description of the Prior Art
Combustion at lean air-fuel ratios is effective to improve fuel economy of automobile internal combustion engines, and such lean air-fuel combustion (lean burn) is actually used in diesel engines and some types of gasoline engines. However, in the lean burn engine, NOx reduction by a three-way catalyst cannot be expected, and therefore, an alternative means for reducing NOx needs to be developed.
As a catalyst capable of reducing NOx under oxidizing conditions of the lean burn engine, Japanese Patent Publication HEI 1-130735 discloses a zeolite catalyst carrying transition metals which can reduce NOx in the presence of hydrocarbons (HC). To supply hydrocarbons to the catalyst, Japanese Patent Publication SHO 63-283727 proposes to provide a particular HC source different from a fuel source and a particular device which introduces hydrocarbons from the HC source into exhaust gas of the engine.
However, provision of such particular HC source and such HC introduction device would increase cost, make the system complicated, and degrade reliability of the system.